The extraction of a solid phase like crystals from saturated liquids is named crystallization. The chemical composition of crystals is different compared to a mother liquid. This characteristic of crystals is used for large scale separation, ultra purification and suspension processes.
In industrial applications crystallization is, after distillation, the most commonly applied technological technique. It includes an extraction of dissolved one component from liquid, the concentration simultaneously of all dissolved substances by solvent crystallization (cryoconcentration), production of required consistence foods (i.e. ice cream).
The over saturated liquid can be prepared by increase of concentration or by cooling. In order to increase the liquid's concentration there are several widely used, commercialized methods including a thermal process, vapor compression, membrane (reverse osmosis), a combination of mentioned processes.
The main disadvantages of membrane's technologies are a high value of energy expenditure for the liquid passing through membranes and their expensive price. The vapor compression equipment and thermal process plants are cumbersome because of very low effectiveness of these heat transfer processes and very high value of energy expenditure for evaporation (597kcal/kg=2500kJ/kg).
The equipment in which the cooling causes a liquid over saturation is named a cooled crystallizer. It is, may be, periodic or continuous type, direct or indirect type.
In generall, the continuous indirect type cooled crystallizer is a well-known scraped surface crystallizer (SSC). It consists of cooled cylinder (evaporator) with a knife arrangement cutting produced crystals from the cooled surface of SSC. If a cylinder revolves, a knife (mill) is fixed. In turn, into an immovable evaporator there is a revolved scraper or shaft with knives. Because of a clearance between cooled surface and the edge of knife or scraper a crystal layer of the treated material covers always a heat transfer surface. It causes a sharp decreasing of heat flow through cylinder and a drop of its capacity. Besides that the adhesion force between knife and a crystal ice layer increases the energy expenditure of a shaft engine and as consequence a cross-section of cylinder is corked up for the passed liquid. In order to prevent this phenomena the construction of SSC is complicated by different defense arrangements. The energy expenditure for these techniques equals about (100 kcal/kg=419 kJ/kg).
An apparatus of freeze crystallization for the removal of water from a solution of dissolved solids, as seen in U.S. Pat. No. 5,575,160 shows that a liquid, passing through a freeze crystallizer, is cooled by direct contact with cooling surface (ice crystal nuclei moved from the inside surface throughout the entire volume of the tubular element). The crystallizer, which converts the initial feed stream into a slurry of ice and concentrate, includes a SSC that produces, pumps and removes an ice slurry and uses a secondary cooling system. A mixture of concentrated liquid and ice crystals, after SSC, is separated and ice crystals are continuously discharged from the ice separator.
However, the method according to the above patent is highly inefficient, because of the clogging of the entire tubular element volume. The scrapers revolve on their shaft causing a mass of liquid to be forced from the shaft in all directions to the cooling surface. The ice crystals, in turn, speed to the axis of the outer shell, due to the existing difference between densities of ice and liquid. New portions of ice continuously press and envelope the revolving shaft. This phenomena causes a decrease of crystallizer's cross-section and a stop of a working mode.
The effectiveness of freezing and the ability to separate ice crystals and mother liquor are dramatically reduced in case of highly contaminated liquid feed streams, because the produced ice crystals are relatively small and therefore a centrifugal action is low.
A further serious disadvantage of the above mentioned method is the fact that the known crystallizer is very inefficient because of the low value of heat transfer coefficient (not more than 400 W/(m.sup.2.multidot.K)) from a coolant (brine) to the feed stream. That's why in comparison to other feed techniques (i.e. pumpless overfeed refrigerant system) this method requires the larger heat transfer surface (by 50.div.100%), constant attention to the stability of the feed stream flow and compressor protection on the refrigerant side.
Moreover, there is large energy consumption by ice forming on the inside surface of the crystallizer's outer shell because an every new frozen ice layer creates additional heat impedance for the next ice layer. It makes it necessary to decrease the refrigerant evaporating temperature. For example, for the partially-crystallized slurry, which may have an ice fraction of 50% from the treated liquid with the crystallization temperature of minus 3.degree. C., it is needed an its final highest temperature of minus 6.degree. C. {[1-(-3.degree. C. /-6.degree. C.)].times.100%=50%}. In case of stainless steel crystallizer's shell and stainless steel body of brine cooler the smallest temperature drop between the treated liquid and boiled refrigerant will be about (15.div.5.20).degree. C. The growing of ice layer on the cooled surface of the crystallizer causes the refrigerant evaporating temperature decrease of about 10.degree. C. At the same time every 1.degree. C. reduction of the evaporating temperature corresponds to 4% cold capacity decreasing. It means that the 10.degree. C. drop of evaporating temperature leads to 40% decreasing of compressor cooling capacity and correspondingly to the 20.div.30% increasing of energy consumption by the compressor.